One other point to add. That is the difference between “>/dev/null 2>&1″ and “2>&1 >/dev/null”

As posted above, “>/dev/null 2>&1″:
- redirects stdout(1) to the file /dev/null
- THEN redirects stderr(2) to stdout(1)

The other, “2>&1 >/dev/null”:
- redirects stderr(2) to stdout(1)
- THEN redirects stdout to the file /dev/null

Now, that all sounds reasonable. It also sound like both statements accomplish the same thing…. but they don’t. The assignment of descriptors is absolute. If you assign stdout to /dev/null, then later assignments follow that same path. If you assign stderr to stdout, all stderr messages will go to the stdout device. If you later assign stdout messages to /dev/null, the messages going to stderr will still get sent to the stdout device because “>/dev/null” doesn’t repoint the device, it repoints the stream.

You don’t believe me. Ok, that;s fine. Bust out gcc and see for yourself and welcome to the funky world of unix descriptors… where 256 is no longer the limit (that’s a descriptor joke).

flinux:~% cat test.c
#include
main()
{
write(1, “hello “, 6);
write(2, “world\n”, 6);
}

flinux:~% gcc -o test test.c
flinuk:~% ./test
hello world
flinux:~% ./test >/dev/null
world
flinux:~% ./test >/dev/null 2>&1
flinux:~% ./test 2>&1 >/dev/null
world

Best Regards,
Kalin Mandaliev

In the C programming language a program has three data streams which can be handled separately :
0 means STDIN (STandarD INput) which is usually the keyboard;
1 means STDOUT (STandarD OUTput) which is the monitor;
2 means STDERR (STandarD ERRor) which is usually also the monitor by default.

A program e.g. X by default prints it’s output to STDOUT, to the monitor.
In the “X >/dev/null 2>&1″ statement, the first part “X >/dev/null” redirects X’s output from STDOUT to the /dev/null file which is something like a “bottomless hole” in Linux. What goes there never comes back.
But X’s STDERR isn’t redirected yet so X’s error messages will still be printed to the monitor.
The last part of the statement “2>&1″ redirects the STDERR (2) to STDIN (1) which is already redircted to /dev/null.
So none of them prints to the monitor any more.
(We have to add the “&” sign before the number 1 otherwise STDERR (2) would be redirected to a simple file called 1 in the same directory instead of STDOUT.)